1. Field of the Invention
The present invention relates to vehicle generators which are used for supplying electrical power to a vehicle for example.
2. Description of Related Art
Generators which are driven by a drive source, for example, an internal combustion engine, are often used to supply power in vehicles. Such generators are often three-phase or multiphase, a phase signal, for example, a U, V or W signal in the form of a generator output signal, being assigned to each phase and output. Most generator output signals form a total multiphase generator signal, which may be fed into a vehicle electrical system after rectification, for example.
Based on a coupling between the drive source, for example, an internal combustion engine, and the generator, each of the generator output signals may be used for detection of the running drive source and thus also an instantaneous generator speed. For example, a generator regulator, which uses one or more generator output signals to analyze a phase frequency, i.e., a frequency of a generator output signal and/or a phase amplitude, i.e., an amplitude of a generator output signal, may be used for this purpose.
For frequency analysis, for example, a change in a generator output signal may be detected, a number of changes in the generator output signal within a predetermined period of time allowing an inference regarding its frequency, so-called phase frequency. The frequency analysis is usually performed by an analysis circuit, for example, a frequency or phase analysis circuit. For detecting a change in a generator output signal, for example, one fixed threshold or two fixed thresholds and a hysteresis may be used, whose difference defines a signal amplitude range. In the case of generator output signals of variable amplitude, variable thresholds may also be provided, as described in published German patent application document DE 19732961 A1, for example. In addition, the frequency analysis may be performed on the basis of a differentiation between generator output signals assigned to two different phases.
Known methods for detecting a frequency of a generator output signal are based on its sinusoidal or square-wave curve. FIGS. 1 and 2 show examples of curves 101 and 201 of such generator output signals. However, based on the development of modern generators having three or more phases, based on mechanical changes or the use of compensation magnets or permanent magnets, a generator output signal to be analyzed may change greatly in the flank area in particular, which may result in an analysis of a phase amplitude or a phase frequency no longer being performable in all frequency and load ranges. FIGS. 3, 4 and 5 show a few curves 301, 401 and 501 of such generator output signals as examples. The flank transitions, which are no longer vibration-free and which sometimes may have high-frequency components, are problematical for the analysis methods known so far.
In addition, one must also take into account the fact that a speed range of a generator may be in a speed band between 0 and 20,000 rpm. This yields a phase frequency of 0 to 2000 Hz for a six-claw pole pair generator or 0 to 2700 Hz for an eight-claw pole pair generator, so that the frequency analysis must often be performed in a wide frequency band. Furthermore, interference signals whose interference frequency may be at low speeds and therefore low-phase frequencies in the case of a useful signal frequency of a generator output signal may be superimposed on the generator output signals at high generator speeds or generator phase frequencies.
FIG. 6 shows as an example a curve 601 of a generator output signal having a voltage amplitude UPhase at low speeds, an interference signal of duration tstör being superimposed on this generator output signal within a time interval 603. A curve 605 of a generator output signal at higher frequencies is shown for comparison; its period 607, which corresponds to a useful duration tNutz, may correspond to interference period 603.
The interference signals also distort the amplitudes of the generator output signals. If the amplitudes of the generator output signals are low at a peak-to-peak voltage of less than 1 VSS, then interference levels whose interference amplitude is greater than 1 VSS have a negative effect on the analysis accuracy of threshold-based frequency analysis concepts.
To analyze the frequency of a generator output signal, for example, a configuration shown in FIG. 7a may be used. A generator output signal, for example, a U, V or W phase component of a generator total output signal, is conveyed to a first comparator 703 and to a second comparator 705 via an input terminal 701. The outputs of comparators 703 and 705 are connected to a detection element 707, which detects whether thresholds predetermined by comparator 703 and 705 have been undershot or exceeded. A first output 709 of detection element 707 indicates that an upper threshold has been exceeded; a second output 711 indicates that the generator output signal has dropped below a lower threshold. A third output 713 indicates that the generator output signal is within the window range predetermined by comparators 703 and 705. An incrementer and decrementer 715 is connected downstream from detection element 707, whose output is connected to a d.c. voltage offset element 717. One output of d.c. voltage offset element 717 is connected to a second input of comparator 705 and to an element 718, whose additional output is connected to another input of comparator 703. The upper and lower thresholds for comparators 703 and 705 may be set via element 718 in advance. Furthermore, an element 719, which transmits a regulator status of a generator regulator to element 718, is also provided. One output of detection element 707 is also connected to a frequency detection element 721, over whose output the phase frequency may be output. Therefore, a change in the generator output signal from which the phase frequency may be derived, for example, may be implemented on the basis of a window comparator having a detector and counter downstream from the latter.
FIG. 7b shows a block diagram of a device for detecting a change in an output signal, which is completely digital. For this purpose, this device, in contrast with the device from FIG. 7a, includes an upstream analog-to-digital converter 719. Furthermore, an element 720, which transmits a regulator status of a generator regulator to element 718, is also provided.